What are the ways to design novel materials for solar energy applications or optimize the existing ones?
A semiconductor suitable for solar cell applications needs to possess two important features. First it has to have a bandgap of around 1.5 eV . Second, the electron-hole pairs that are created as a result of the incident light should have long lifetimes. The aim of my research is to study transition metal oxides and their alloys using ab-initio methods and see how their excited state properties can be manipulated to achieve the aforementioned conditions. Specifically, I work on Nickel Oxide, which is already used in solar applications: it is an effective co-catalyst to produce hydrogen in photocatalysis. Its problem is its large bandgap. Alloying with different materials is a strong candidate to reduce this gap. My work is to use methods such as DFT+U and hybrid DFT to see which one reproduces the ground state properties of pure NiO more accurately. The ground state method is then used to study the effect of alloying on ground state properties and also to optimize the structures that are to be used for excited state calculations. The final aim is to calculate the optical bandgap and carrier lifetimes. The methods used for the latter are Embedded Configuration Interaction and GW. As an improvement on point charge embedding methods that are already used in our group, I am working on including the effects of the polarization of surrounding atoms (which are represented by point charges) using the Shell Model for atomic polarization. Another aim of my project is to calculate conductivity using methods that are being developed by other members of our group. (Back)
How to use orbital-free density functional theory to study liquid metals?
Orbital-free density functional theory (OFDFT) is a powerful linear-scaling method for electronic structure calculations. Instead of calculating individual Kohn-Sham (KS) orbitals, as in KS-DFT, OFDFT uses the electron density as the sole working variable, and thus, is capable of simulating large systems containing millions of atoms. With the recent progress in OFDFT methods, OFDFT is no longer restricted to light metals but can also be applied to semiconductors, transition metals, or molecules. In my study, I will carry out ab-initio molecular dynamics to investigate various properties of liquid metals. These are used as important plasma-facing materials in fusion reactors.The molecular dynamics calculations will be based on newly-developed techniques in OFDFT, such as small-box Fast Fourier Transforms (SBFFT). While traditional FFT implementations scale poorly for more than a few hundred processors, SBFFT enables us to do highly parallelized calculations using tens of thousands of cores to do FFT. The latter will be necessary for large-scale molecular dynamics simulations. (Back)
How to combine CW methods and DFT methods for higher efficiency and accuracy?
A quantitative understanding of a chemical process requires a balanced consideration of efficiency and accuracy. Density functional theory (DFT) scales well with system size, and can readily describe extended systems by the use of periodic boundary conditions, to obtain, e.g., bulk properties. Unfortunately, DFT is limited to ground-state calculations and the use of approximate exchange-correlation functionals. More sophisticated ab-initio correlated wavefunction (CW) methods feature a better description of electronic correlation and excited states. However, the prohibitively large computational cost severely limits their applicability. Furthermore, combining the exact treatment of correlation with the use of periodic boundary conditions is challenging. One possible strategy to overcome these restrictions is to partition a larger system into parts of manageable size that can be treated on different levels of theory: one describes the region of interest (cluster) via a CW method, while the environment is treated with DFT. Such an embedding scheme should reproduce the electronic structure of the cluster while reducing the computational costs to an acceptable level. Our group has developed a potential-functional-based embedding theory [ J. Chem. Phys., 135, 194104 (2011)] for this purpose, that (1) avoids ambiguities by introducing a global embedding potential to model the interaction between subsystems, (2) allows for flexible combination of different methods (DFT, CW), and (3) can be done in a self-consistent fashion. Embedding applications include the interaction of gas molecules with metal surfaces (CO on copper, O2 on Aluminum), that require both a high-level treatment at the adsorption site as well as a description of the extended surface. In the future, the CW/DFT embedding scheme will be used to study the mechanism of a chemical process, e.g., the oxygen reduction reaction on the cathode material of solid oxide fuel cells. (Back)
How to develop local MRCI methods and global optimization techniques?
Local approximation in multireference methods are carried out to facilitate the study of larger chemical systems as occurring in, e.g., biodiesel combustion processes. Within this linear-scaling theoretical framework, additional performance gains can only be realized through parallelization and micro-optimizations. These efforts are in line with developing new features within the TigerCI code empowering new research based on large-scale multireference calculations.
Optimization problems play a pivotal role in theoretical chemistry. Since most of them prove to be non-deterministic polynomial-time hard in reality, their solution requires the application of efficient global optimization techniques. Through the development and interfacing of global optimization techniques based on genetic algorithms, our research targets problems such as basis set optimization and structure optimizations. (Back)
How can we use quantum methods to design new photocatalytic materials for production of solar fuels?
Photocatalysts can be used for solar fuel production by inducing water splitting to produce H2 and O2 or by catalyzing CO2 reduction by water to produce O2 and small hydrocarbons. We will use quantum approaches to design new affordable materials with optimal photocatalytic properties, such as correct band gap width and position and long excited state lifetimes. New materials studied will be biologically motivated, by looking at complexes used in nature to catalyze related reactions and choosing similar metal oxides or sulfides. The challenge lies in choosing the correct theoretical technique for predicting different properties. Ground state properties will be determined using Density Functional Theory (DFT), or an ab initio DFT+U approach if appropriate for the material. Localized excited electronic states will be calculated using our embedded configuration interaction (ECI) theory, and delocalized excited states will be calculated using the Green’s functions techniques of the GW approximation and the Bethe-Salpeter equation. ECI will also be used to calculate the chemistry of the active site, by modeling the charged cluster and determining the barriers and rate constants for redox reactions. With this approach, we should be able to use quantum mechanics to predict a photocatalytic material’s excitonic states and describe subsequent chemical reactions. (Back)
Can we recycle CO2 into liquid fuels?
Two great challenges facing science in the 21st century are finding ways to address increasing energy consumption demands amid declining fossil fuel reserves and to reduce atmospheric CO2 accumulation from emissions. Understanding the fundamental process of catalytic conversion of CO2 into methanol should unlock new technologies to recycle CO2 emissions into liquid fuels and/or useful products, in principle addressing both problems at once. Although CO2 reduction to methanol is highly unfavorable thermodynamically, experimental collaborators at Princeton University have promising results showing pyridinyl-catalyzed electrochemical CO2 reduction is possible under relatively low electrode overpotentials, low enough that this chemistry can be powered by sunlight. How this 6-electron transfer process occurs and how to maximize its efficiency is not well understood, however. Using first-principles computational methods, we are elucidating the reaction mechanisms for CO2 reduction, studying its canonical reaction mechanism in solution as well as at electrode/liquid interfaces. By understanding the individual reaction steps and their dependence on environmental factors, mechanistic landscapes can mapped out which would provide a template for how CO2 reduction catalysts can be engineered. (Back)
How to expand the usefulness of reduced scaling multireference configuration interaction methods?
In principle, full Configuration Interaction (CI) provides an exact solution to Schrodinger's equation in a finite basis set. In practice, full CI is too computationally expensive to investigate the chemistry of all but the smallest of molecules. The computational cost can be reduced by truncated CI methods; however these methods do not provide an exact solution. Recently members of the Carter group have proposed a series of methods employing local electron correlation to further reduce the computational cost of CI truncated at the singles and doubles level (SDCI). These local correlation methods remove terms which describe negligible nonlocal electron correlation while retaining the accuracy of the SDCI energy. The goal of our research is continue to expand the usefulness of our local correlation methods. We plan to develop fast algorithms for incorporating size extensivity corrections, calculate transition dipole moments, and energy gradients and further increase the size of molecule we can investigate by parallelizing our computer code. (Back)
How can we convert CO2 into liquid fuels and chemicals?
Identifying renewable energy sources and reducing atmospheric CO2 require immediate attention. Photocatalytic reduction of CO2 to useful products may handle both of these issues. The aim of my research is to study this catalytic process using first principles quantum mechanics methods. In particular I will consider the semiconductor gallium phosphide (GaP) as a photocatalyst. I will investigate the interactions between its surface and the species participating in the CO2 reduction. In addition to the solid catalyst, recent studies suggest that a co-catalyst formed from a pyridinium ion might play a crucial role in achieving efficient CO2 reduction over the semiconductor surface. As part of my research, I will work on elucidating the reaction mechanism involving the pyridinium ion, CO2, water, and the GaP surface that leads to CO2 reduction. (Back)
Can the kinetics of biodiesel combustion be modeled quantitatively using multireference wavefunctions?
In order for the nation to maintain its edge in economic competitiveness, uphold its standard of living, and ensure its national security, alternatives to fossil fuels must be developed. Fossil fuels currently account for as much as 85% of our energy uses and viable, secure, and economical alternative energy sources must be created. A promising but poorly understood renewable alternative fuel for the transportation sector (our second largest use of energy after power generation) is biodiesel. Biodiesel is a non-petroleum-based fuel composed of monoalkyl esters obtained from long-chain fatty acids derived from renewable lipid sources by transesterification with methanol. We plan to develop and apply validated ab initio methodology to quantitatively predict the kinetics of important biodiesel combustion reactions with the goal of better understanding their mechanisms as they burn in internal combustion engines. (Back)
Why does biodiesel burn more cleanly than petro-diesel?
Biodiesel is an oxygenated fuel principally comprised of saturated and unsaturated methyl esters. During combustion, the functional moiety, O-(C=O), in biodiesel esters promote complete oxidation, resulting in low levels of soot production. While the ester moiety reduces soot formation, experimental studies have shown that the C=C double bonds in unsaturated methyl esters promote soot formation. My research uses ab initio methods of quantum chemistry to understand how the ester functionality in biodiesel fuels leads to low levels of soot production and to resolve the role played by the C=C bond in unsaturated esters in reducing this soot reduction benefit. (Back)
How can we design catalysts that convert CO2 into liquid fuels?
In my research I am investigating water oxidation and CO2 reduction with the help of a variety of catalysts. We are designing transition metal oxides (TMOs) that exhibit a band gap which is suitable for light absorption in the visible spectrum. The excited electrons can be used to reduce CO2 and the holes can oxidize water. In my research I am modelling all important processes that occur on the surface of the TMOs, e.g. the adsorption of water and CO2 molecules on the surface and cleavage of the C-O bond in CO2 and the O-H bond in water. With our findings we want to understand the mechanisms that govern heterogeneous CO2 reduction and water oxidation and get insight into important properties for the design of new photocatalysts. Complementary to the heterogeneous catalysis by TMOs, a second approach to reduce CO2 uses homogeneous catalysis by a rhenium complex. This complex was experimentally shown to reduce CO2 with high efficiency and selectivity. A similar complex, using much more abundant manganese instead of rhenium, has been shown to exhibit similar catalytic properties, and at the same time working at even less overpotential. In close collaboration with experimentalists we are investigating the redox properties of both complexes and the mechanistic properties of the catalytic reaction. (Back)
Can we design improved cathode materials for solid oxide fuel cells?
Solid oxide fuel cells (SOFCs) are electrochemical devices which potentially may help to meet our energy demands in the future. A SOFC directly converts the energy stored in fuels such as hydrogen and light hydrocarbons into electrical energy. The factor that limits the performance of existing SOFC devices is the kinetics of the oxygen reduction reaction occuring at the cathode. The oxygen molecules in the gas phase must react to become oxygen ions in the bulk of the solid oxide electrolyte. This reaction involves a complex set of steps with generally undetermined kinetics including adsorption, diffusion and reduction rates. The goal of our research is to use ab initio quantum mechanical methods to understand how material composition determines the reaction rates of the oxygen reduction reaction (and in turn the cathode performance). Through understanding how the best existing materials operate, we will seek to find materials which improve SOFC cathode performance. The results of the research will serve as the starting point for higher level modeling including kinetic monte carlo methods. (Back)
Ilgyou (Isaac) Shin
How to study metal alloy microstructural properties with orbital-free density functional theory?
In the past decades scientists have been trying to develop both fast and accurate methods to study materials. However, conventional first principles electronic structure methods are still time consuming even for a few hundred atoms and empirical potentials are not transferable enough to investigate systems outside the regime to which they were fit. In order to study properties of materials both quickly and accurately, OF-DFT (Orbital-Free Density Functional Theory) can be a satisfying choice combining the accuracy of DFT with the computational efficiency of the OF approach. I am studying mechanical and structural properties of aluminum and magnesium alloys for lightweight vehicles applications with OF-DFT as well as the theory itself. (Back)
How do oxygenated biofuels perform differently from conventional hydrocarbon based fuels?
The typical feature of biofuels which distinguishes them from conventional hydrocarbon fuels is the oxygen atoms included as an additional element in the molecular constitution. The presence of the oxygen atoms in biofuels changes the properties of these fuel molecules, and then makes their combustion behavior different from familiar hydrocarbon fuels. Their chemical decomposition and oxidation pathways are well coupled to the structure of the corresponding fuel molecules. Predicting the combustion behavior of these fuels requires the development of detailed combustion mechanisms, which must include quite a large number of reactions, rate coefficients, as well as related thermochemical and transport parameters. I am using the ab initio multireference configuration interaction method to find out the effect of these oxygen atoms on the fuel molecules, and to improve the accuracy of the parameters which are necessary to define the combustion mechanism. (Back)
Junchao (Steven) Xia
How to extend orbital free density functional theory to more and more broad areas?
Orbital free density functional theory (OFDFT) is an efficient first principles theory that has the potential to provide an effective approach to study large scale material properties. However, it is most reliable only in light metal systems, limiting its applicability to many other interesting scientific phenomena. My current research is to develop and improve OFDFT, particularly for covalent systems. An accurate as well as fast formalism is desired large scale covalent material samples can be studied with OFDFT. Once it is achieved, we can study mechanical or other properties on not only light metals, but also semiconductors or even with molecules with OFDFT with very large numbers of atoms that other first principles theories cannot handle. The ultimate goal is to extend OFDFT to more and more broadly applicable areas and simulate various interesting situations. (Back)